In vertebrates, the endocrine system includes a set of glands and hormones produced by these glands to regulate various functions and metabolism. Endocrine glands release its hormones directly into the blood.
What are Hormones?
They are chemical messengers or informers which are carried by blood from the site of secretion to the site of action for a specific biological function. They could be either amine or protein or steroid in nature. Their production could be stimulated either by intrinsic factors or by extrinsic factors. Once hormones are released, they are transferred from the production site to target site via blood. They are target specific i.e. they will bind only to those receptors which are specific to them. Once they lock with the target tissue, necessary changes will take place.
Why hormones are called chemical messengers?
The prominent role of hormones is that of a messenger. Hypothalamus is a part of forebrain where a numerous amount of neurosecretory cells are present. These neurosecretory cells are specialized in the secretion of a hormone called neurohormones. They stimulate the anterior lobe of the pituitary to produce various other hormones.
Sometimes, hormones act more as a regulator than a messenger. The changes in the level of hormone production lead to certain changes in the body. Thus, hormone as a regulator maintains the homeostasis of the body. Once the hormones meet their target, their production needs to be controlled and this is attained by a mechanism called feedback control mechanism. The feedback mechanism could either be positive or negative.
Feedback Mechanism – Thyroid
For example, the Thyroid gland produces a hormone called thyroxine, and its secretion is controlled by the Thyrotropin Releasing Hormone (TRH) from the hypothalamus and the Thyroid Stimulating Hormone (TSH) from the anterior pituitary.
When the level of thyroxine in the blood reduces, hypothalamus stimulates the thyroxine secretion by stimulating TSH secretion. This represents a positive feedback mechanism.
If hypothalamus continues to stimulate thyroxine production, it could result in high level of thyroxine in blood. This sends a negative feedback to the hypothalamus to reduce or stop the TRH and TSH secretion which regulates the thyroxine level in the body. This is the negative feedback mechanism.
Hormones are meant for their target tissues for specific functions. As soon as they meet their target, they are removed. This is mainly done by the liver, kidney and other organs.
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